1. Field of the Invention
The present invention relates to a light-emitting device with the use of a diamond, for emitting ultraviolet light, that can be used in fields of optical information recording/read out processing, photolithography, optical working, fluorescent light sources, etc.; more specifically, a current injection-type diamond ultraviolet light-emitting device that emits ultraviolet light due to excitation by the injection of current.
Since the ultraviolet light has a short wavelength and find processing can be performed with this light, various demands for this light have been increasing. For example, there are applications in increasing memory density by using the light for optical recording/read out processing, increasing packing density by using the light for semiconductor find processing equipment, etc.
2. Description of the Related Art
For sources of this ultraviolet light, one may point out a deuterium lamp, an excimer laser, etc. However, the deuterium lamp has a poor efficiency in ultraviolet emission and low brightness. Further, since the excimer lasers employ gases, they are of a large size and need water-cooling and hence inconvenient to handle with. In addition, some excimer lasers employ hazardous substances (halogen). As described above, the use of conventional ultraviolet light sources causes much inconvenience.
Further, a diamond is also known to be a material capable of emitting the ultraviolet light. The diamond ultraviolet light-emitting device is of a small size, efficient, highly bright, and also excellent in safety.
Conventional diamond light-emitting devices are described in, for example, (1) Japanese Patent Application Laid-open No. 4-240784, (2) Japanese Patent Application Laid-open No. 7-307487, (3) Japanese Patent Application Laid-open No. 8-330624, etc.
However, in the conventional diamond light-emitting devices, diamond crystals comprising luminescent layers have poor quality, containing many impurities and defects therein. Due to this fact, ultraviolet emissions originating in the impurities and lattice defects are dominant, and therefore free exciton recombination radiation of a short wavelength that is intrinsic to the diamond and observable only when the impurities and defects are sufficiently reduced is not dominant. For example, in the embodiment (FIG. 9) of the Japanese Patent Application Laid-open No. 7-307487, a peak of the 238 nm wavelength is dominant. This particular emission has been determined to be a recombination radiation of an exciton bounded at boron, hence originating in the impurity still. Therefore, conventional diamond light-emitting devices bear problems to be solved as below. That is,
(1) When the diamond light-emitting device is constructed as a current injection-type light-emitting device, ultraviolet emission originating in the impurities or lattice defects is dominant and the free exciton recombination radiation of a shorter wavelength (wavelength of 235 nm etc.) that is intrinsic to the diamond and most advantageous in a practical point of view cannot exhibit sufficient emission intensity. Therefore it is impractical to use it as an actual light-emitting device.
(2) Further, the conventional diamond light-emitting devices are considered on the basis of the ultraviolet emission originating in the impurities or lattice defects, and therefore the amount of dopant is examined only in terms of the control of the conductivity of the crystal. Consequently, nothing has been examined regarding the existence of the allowable maximum amount of a dopant for creating the conductivity of the diamond that does not hinder the free exciton recombination radiation effectively.
(3) Further, since the conventional diamond light-emitting devices are dependent on the ultraviolet emissions originating in the impurities and lattice defects, a composition and a growth sector of the diamond crystal have been poorly investigated and optimization of its crystallinity and the controlling of wavelength based on alteration of isotopic composition ratio of the diamond have not been tried.
Therefore, it is an object of the present invention to solve the above-described problems and provide an injection current-operated diamond ultraviolet light-emitting device in which the free exciton recombination radiation intrinsic to the diamond and having a shorter wavelength is dominant.
With an aim to achieve the above-described object, the diamond light-emitting device according to the present invention is a diamond ultraviolet light-emitting device comprising a luminescent layer composed of a diamond capable of emitting light at a given wavelength due to excitation by the injection of current, characterized in that the diamond light-emitting device has a luminescent layer comprising a diamond in which occurs an emission such that free excitation recombination radiation due to excitation by the injection of current is dominant. Here, a diamond in which occurs an emission such that the free exciton recombination radiation by the injection of current is dominant means a high-quality diamond with few impurities and few lattice defects except dopants. Further, an emission such that the free exciton recombination radiation is dominant means emission whose emission intensity originating in the free exciton recombination is at least twice as large as the emission intensity originating in the impurities.
Preferably, such a diamond comprises a diamond crystal fabricated by the high-temperature and high-pressure method using a flux with an impurity getter added in. Here, the above-described impurity getter is preferably a nitrogen getter. The reason for this is that nitrogen in the diamond, in addition to the lattice defects, adversely affects the free exciton recombination radiation in particular. Therefore, it is desirable that the amount of nitrogen contained in the diamond crystal is controlled to be no more than 10 ppm by means of the nitrogen getter.
Preferably, the above-described diamond is such that the ratio of the peak intensity of the free exciton recombination radiation to that of the visible emission in its photoluminescent spectrum at room temperature is larger than 0.1 or such that the full width at half maximum of its Raman scattering peak intrinsic to the diamond is no more than 1.9 cmxe2x88x921. The reason of this condition is to prevent impurities and lattice defects from being included in the crystal, because they (=impurities and lattice defects) create the other emission peaks and at the same time scatter the free exciton, hence reduce its recombination radiation intensity.
Preferably, the above-described diamond is a p-type semiconductor containing boron, and further preferably, the boron content is no more than 40 ppm as measured through the infrared spectroscopy, and furthermore preferably, the boron content is no more than 2xc3x971019 atoms/cm3 as measured through the SIMS(Secondary Ion Mass Spectroscopy) method. The reason of this condition is that such a range is considered to be a desirable upper limit for the concentration of boron in terms of increasing the conductivity and at the same time suppressing the boron-originated emission intensity.
Preferably, the above-described diamond has a given carbon isotope composition ratio that enables the emission at the above-described given wavelength, for example, the ratio 12C: 13C is in the range from 1:99to 99:1. To describe this effect specifically, when the purity of 13C isotope is 99%, the peak wavelength of the emission can be shifted to a shorter wavelength by approximately 1 nm compared to a case where the purity of 13C isotope is 1%. Therefore, the above-described diamond has wavelength tunability in accordance with the carbon isotope composition ratio.
Preferably, the above-described diamond is a diamond crystal having the {100} growth sector. The reason of this condition is that the {100} growth sector has lower concentrations of the impurities and lattice defects compared to those grown on other growth sectors and high-quality.
Preferably, the above-described diamond is a diamond crystal such that an electric conductor layer is formed on its surface, and more preferably, such that the aforesaid electric conductor layer is a diamond crystal formed by surface hydrogen-termination treatment. The reason of this condition is that thereby the conductivity of the diamond crystal is improved.
Therefore, in the following description, xe2x80x9chigh-qualityxe2x80x9d means to fulfil at least one item of six items below. (1) The impurities and lattice defects are small in number except dopant(s) and there occurs an emission in which the free exciton recombination radiation is dominant. (2) Preferably, the nitrogen content in the diamond crystal is no more than 10 ppm, which is achieved through the fabrication by the high-temperature and high-pressure method with the use of a flux with an impurity getter (for example, a nitrogen getter) added in. (3) Selection of diamonds is performed in consideration of a condition that the ratio of the peak intensity of the free exciton recombination radiation to that of the visible emission in its photoluminescence spectrum at room temperature is larger than 0.1 or a condition that the full width at half maximum of the Raman scattering intrinsic to the diamond is no more than 1.9 cmxe2x88x921. (4) The diamond is a p-type semiconductor containing boron, and preferably, the boron content is no more than 40 ppm as measured through the infrared spectroscopy, and further preferably no more than 2xc3x971019 atoms/cm3 as measured through the SIMS method. (5) Regarding the growth sector dependence, preferably the diamond is grown on the {100} growth sector. (6) An electric conductor layer is provided on the surface of the diamond crystal as an electric conductive mechanism so that dopant can be dispensed with (for example, the hydrogen-termination treatment of the surface of the diamond crystal).